[PATCH] kho: do not reserve empty per-node bootmem areas

[Mike Rapoport]

On Wed, Oct 14, 2026 at 03:15:44PM +0200, Pratyush Yadav wrote:
> From: "Pratyush Yadav (Google)" <pratyush at kernel.org>
> 
> Some nodes do not allocate any early boot memory. An example is
> hotpluggable nodes with movable_node enabled. So the scratch size for

                          Let's use the new naming please ^ ;-)

I'll fix it up when applying.

> them ends up being zero. Zero size allocations normally don't play well
> with memblock. Memblock returns an address, but then doesn't actually
> reserve anything. This address ends up being recorded in kho_bootmem
> with a size of 0. In case of hotpluggable nodes with movable_nodes, the
> allocation on the node actually fails because memblock refuses to
> allocate there, and so KHO fails to initialize.
> 
> Skip bootmem reservation for such nodes entirely.
> 
> To keep the code more robust, avoid overloading kho_bootmem_cnt. use
> max_nr_bootmem to store the size of kho_bootmem array. This ensures that
> the location of kho_bootmem_cnt assignment does not affect the cleanup
> paths.
> 
> Fixes: 3dc92c311498 ("kexec: add Kexec HandOver (KHO) generation helpers")
> Reported-by: Bing Jiao <bingjiao at google.com>
> Signed-off-by: Pratyush Yadav (Google) <pratyush at kernel.org>
> ---
>  kernel/liveupdate/kexec_handover.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> index 7018e25e4eda..6f1f55fe3181 100644
> --- a/kernel/liveupdate/kexec_handover.c
> +++ b/kernel/liveupdate/kexec_handover.c
> @@ -831,8 +831,8 @@ bool kho_bootmem_overlap(phys_addr_t phys, size_t size)
>   */
>  static void __init kho_reserve_bootmem(void)
>  {
> +	int nid, i = 0, max_nr_bootmem;
>  	phys_addr_t addr, size;
> -	int nid, i = 0;
>  
>  	if (!kho_enable)
>  		return;
> @@ -840,8 +840,8 @@ static void __init kho_reserve_bootmem(void)
>  	bootmem_size_update();
>  
>  	/* FIXME: deal with node hot-plug/remove */
> -	kho_bootmem_cnt = nodes_weight(node_states[N_MEMORY]) + 2;
> -	size = kho_bootmem_cnt * sizeof(*kho_bootmem);
> +	max_nr_bootmem = nodes_weight(node_states[N_MEMORY]) + 2;
> +	size = max_nr_bootmem * sizeof(*kho_bootmem);
>  	kho_bootmem = memblock_alloc(size, PAGE_SIZE);
>  	if (!kho_bootmem) {
>  		pr_err("Failed to reserve bootmem array\n");
> @@ -882,6 +882,9 @@ static void __init kho_reserve_bootmem(void)
>  	 */
>  	for_each_node_state(nid, N_MEMORY) {
>  		size = bootmem_size_node(nid);
> +		if (!size)
> +			continue;
> +
>  		addr = memblock_alloc_range_nid(size, BOOTMEM_ALIGNMENT_BYTES,
>  						0, MEMBLOCK_ALLOC_ACCESSIBLE,
>  						nid, true);
> @@ -895,13 +898,19 @@ static void __init kho_reserve_bootmem(void)
>  		i++;
>  	}
>  
> +	/*
> +	 * Nodes that needed no bootmem were skipped, so fewer areas than
> +	 * initially estimated may have been reserved.
> +	 */
> +	kho_bootmem_cnt = i;
> +
>  	return;
>  
>  err_free_bootmem_areas:
>  	for (i--; i >= 0; i--)
>  		memblock_phys_free(kho_bootmem[i].addr, kho_bootmem[i].size);
>  err_free_bootmem_desc:
> -	memblock_free(kho_bootmem, kho_bootmem_cnt * sizeof(*kho_bootmem));
> +	memblock_free(kho_bootmem, max_nr_bootmem * sizeof(*kho_bootmem));
>  err_disable_kho:
>  	pr_warn("Failed to reserve bootmem, disabling kexec handover\n");
>  	kho_enable = false;
> 
> base-commit: e1fe7daedae7a4a23aca6dbd58e1746750b632dc
> -- 
> 2.56.0.385.gd3acb90ef8-goog
> 

-- 
Sincerely yours,
Mike.